Known magnetoelastic transducers (see, for example U.S. Pat. No. 2,895,332) consist of a sheet package which is arranged to receive a certain magnetic flux configuration with the aid of excitation windings. When the transducer is mechanically loaded, this magnetic flux configuration is changed, and the change is detected by means of one or more measuring windings. The various windings may be mounted in the sheet package of the transducer, in which case they are arranged to pass through holes in the sheet package, or they may surround the sheet package.
These known magnetoelastic transducers have a large power consumption, especially in the case of transducers which are designed to withstand great loads. The large power consumption is due to the fact that the transducer must be excited to a substantial degree. The large volume that has to be excited also contributes to the need to use only relatively low frequencies for the supply voltage, since higher frequencies result in even greater losses. A high supply frequency is, however, desirable, since it is then possible to measure more rapid force variations. In addition, the components included in associated electronic equipment, for example capacitors, will be less expensive. One way of reducing the power consumption is to influence the magnetic flux configuration in the transducer by changing the configuration of the holes which receive the windings. However, this method only gives relatively marginal effects on the power consumption.
Another disadvantage of the known magnetoelastic transducers is that the power requirement varies with, and is approximately proportional to, the size of the transducer. For example, in the case of force-measuring transducers, the size is approximately proportional to the nominal force or load, and therefore large transducers require a large supply power, which involves the provision of expensive supply equipment. The supply devices and the signal processing equipment are also more complicated if large variations in signal levels have to be dealt with.
By reducing the volume of the magnetic material, the electric power demand will decrease. This reduction in material can be accomplished by replacing a certain number of the sheets of magnetic material with sheets of non-magnetic material. This is entirely feasible and it has been found that up to 99.5% of the sheets can be replaced by non-magnetic material. This way of decreasing the magnetic mateial is described in, for example, Swedish published patent application No. 339,125. However, non-magnetic sheet material, for example of austenitic stainless steel type, has a coefficient of expansion which differs considerably from the coefficient of expansion of magnetic sheet metal, for example silicon steel. Also, the coefficients of elasticity of the two materials are different. For transducers with mechanically assembled sheet packages, such as those disclosed in Swedish published patent application No. 399,125, or with glued sheet packages, this results in a very high temperature dependence for both the neutral point and the sensitivity of the transducer. In order that a glued package of sheets with different properties (modulus of elasticity and coefficient of expansion) shall not crack when exposed to varying load and temperatures conditions, it is a condition that the coefficients of elasticity and the coefficients of expansion of the materials must not be too different. Thus, it has been determined that the coefficient of elasticity of the non-magnetic replacement material should not vary by more than .+-.20% from the coefficient of elasticity of the magnetic sheet material used, and that the coefficient of expansion of the non-magnetic replacement material should not deviate by more than .+-.25% from the coefficient of expansion of the magnetic sheet material. These material requirements make it difficult, at the present time, to find inexpensive replacement materials. Examples of materials that have proved to be suitable are the materials known under the Registered Trade Marks "INCONEL" and "NIMONIC", but these are very expensive materials.
The present invention aims to provide a magnetoelastic transducer of the kind referred to which has a reduced amount of magnetic material without giving rise to the problems mentioned above.